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RE: turning off charge balance

Dr. Parkhurst,

Thanks for your timely response.  I am aware of that example.  However, when
using my inverse model to regress on "best" equilibrium constants, sets of
measured data are used as dependent variables (including pH) and are assumed
to contain a bit of error.  In a hypothetical case where datasets used for
basing the regression are not exactly defined to zero charge balance, it
would be advantagous for the user to manually turn off charge balance during
regression.  In such cases where datasets are closely defined to within a
few percent of charge balance, if charge balance was turned off regression
could occur on dependent variables including pH.  Currently, the user must
define pH exactly as in example 8 and are not able to regress on pH in such


Jon Schwantes
Environmental & Water Resources Engineering
Civil Engineering Department, MS-3136
Texas A&M University
College Station, Texas 77843-3136
h: 979-695-8264, w: 979-845-9772
e: plutonium@xxxxxxxx

"That that you give to others, does not die with you"

-----Original Message-----
From: David L Parkhurst [mailto:dlpark@xxxxxxxx]
Sent: Friday, April 27, 2001 8:35 AM
To: plutonium@xxxxxxx
Subject: Re: turning off charge balance

> I am currently working on an inverse model that uses PHREEQC to back
calculate equilibrium and kinetic constants.  In order for my model to
work, an input file describing several reactor solutions (in the case of EQ
constants) is described and saved and then equilibrated with several
unknown solids.  In the first equilibration, we charge balance on a
fictitious species.  However, during equilibration with the solids, pH is
changed to satisfy charge balance.  Is there a way we can "turn this off"?
The general idea for my model is that it will call PHREEQC to perform
equilibrium several times in order to regress on the real values of the
constants for specified unknowns.  To do this, we regress on several
dependent variables, including pH, so it is imperative that pH is not
affected by charge imbalance.

There is a way to fix the pH, which is demonstrated in example 8 of the


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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